Transmitters or transceivers often use a power amplifier to increase the amplitude of a radio frequency (RF) signal that is provided to an antenna for transmission. In practice, the power amplifier is not perfect, that is, the power amplifier exhibits non-linearities and experiences gain variations, for example, based on changes in ambient temperature or battery voltage. Thus, many transmitter systems implement various closed-loop control techniques, such as output power control or predistortion, to compensate for the non-linearity and gain variation of the power amplifier. In general, these techniques determine an error signal by measuring or sensing the output of the power amplifier and comparing the digitized output of the power amplifier to a digital reference signal based on the baseband input to the transmitter. Based on the error signal, the system adjusts the signal level and/or phase of the input signal to the power amplifier to compensate for any non-linearity of the power amplifier.
Most current systems utilize an analog-to-digital converter to convert the output of the power amplifier to a digital signal for comparison to the digital reference signal. In order to cover the full dynamic range of the output of the power amplifier, the resolution of the analog-to-digital converter is generally around ten to twelve bits or more. As a result, the area and current drain of the analog-to-digital converter limits the ability of a system designer to incorporate these closed-loop power control methods in smaller portable devices, such as cellular phones and other wireless devices, where size and power consumption (or battery life) are at a premium.